Magneto-optical recording medium and head unit for a magneto-optical recording medium

ABSTRACT

A magneto-optical recording medium having a substrate and magneto-optical recording film. The substrate is formed from a material having light transmittivity. One surface of the substrate is provided with a magneto-optical recording film. When light having a wavelength of λ is irradiated on the surface of the substrate having the magneto-optical recording film, grooves are formed in film which have an optical depth with a lower limit of λ/8 and an upper limit of λ/4 or less. The magneto-optical recording film is formed so that the Kerr ellipticity thereof becomes 0°. A head unit for the magneto-optical recording medium has an optical-axis device for compensating the Kerr ellipticity of the magneto-optical recording medium. The optical axis device is in an optical path between an object lens for focusing a light beam emitted from a light source on the magneto-optical recording film and a polarization beam splitter.

TECHNICAL FIELD

The present invention relates to a magneto-optical recording medium anda head unit for a magneto-optical recording medium. Particularly, thepresent invention relates to a magneto-optical recording medium using asubstrate where grooves are formed and also a head unit for amagneto-optical recording medium which compensates the Kerr ellipticityof the magneto-optical recording medium.

BACKGROUND ART

In the magneto-optical disks as a magneto-optical recording medium,grooves for guiding a light beam irradiated to the magneto-optical diskhave hitherto been formed concentrically or spirally. At the time of therecording or reproduction of the magneto-optical disk, the trackingcontrol of the light beam irradiated to the magneto-optical disk isperformed based on the reflected light from the grooves of the lightbeam which was irradiated on the magneto-optical disk. As a consequence,in the magneto-optical disk, data is recorded on an area (hereinafterreferred to as a land) between grooves at the time of the recording, anddata recorded on the land is read out at the time of the reproduction.

Incidentally, in an magneto-optical disk such as this, with the objectof enhancing the recording density there has been proposed amagneto-optical disk where signals are recorded on both a groove and aland and thereby a wide recording area is assured. When amagneto-optical disk such as this is reproduced, however, cross talkfrom nearby grooves or lands becomes a problem.

As a method of reducing this cross talk, there has been proposed anoptical method and a signal processing method. In the optical method,three light spots are irradiated on the magneto-optical disk, and whenthe signal on the groove (or land) is reproduced with the central lightspot, the signal on the land (or groove) is reproduced with the lightspots before and after, thereby eliminating the cross talk caused by thesignal component of the land (or groove) included in the reproducedsignal. On the other hand, in the signal processing method, the crosstalk included in the reproduced signal can be eliminated by improvingthe signal-to-noise ratio (SN ratio) of the reproduced signal withViterbi decoding.

However, in the aforementioned optical method, since three spots areirradiated on the magneto-optical disk and the signals on adjacent landsor grooves are reproduced, there is the problem that the pick-upstructure becomes complicated and is increased in size. Also, the signalprocessing method has disadvantages in that the circuit structurebecomes complicated. Thus, either case has still been unsatisfactoryfrom practical use as a solution of eliminating the cross talk.

DISCLOSURE OF INVENTION

The present invention relates to a magneto-optical recording medium anda head unit for a magneto-optical recording medium which solve theaforementioned problems.

In the present invention, a magneto-optical recording medium has asubstrate formed from material having light transmittivity and also hasa magneto-optical recording film provided on one surface of thesubstrate. In said one surface of the substrate where themagneto-optical recording film is provided, when a wavelength of a lightbeam, irradiated to the magneto-optical recording medium, is λ, thereare formed grooves having an optical depth where its lower limit is λ/8and its upper limit is λ/4 or less.

Furthermore, in the present invention, a head unit for a magneto-opticalrecording medium having a magneto-optical recording film having a Kerrellipticity of a predetermined value, comprises: a light source; anobject lens for focusing a light beam, emitted from the light source, onthe magneto-optical recording film of the recording medium; firstoptical split means for separating a return light reflected from themagneto-optical recording film on which the light beam is incidentthrough the object lens, from the light beam emitted from the lightsource; second optical split means for splitting the return lightreflected from the magneto-optical recording film on which the lightbeam is incident through the object lens, separated by the first opticalsplit means, into a P-polarized light component and an S-polarized lightcomponent; and compensation means disposed in an optical path of thereturn light between the object lens and the second optical split means,for compensating the Kerr ellipticity of the magneto-optical recordingfilm of the recording medium.

In the magneto-optical disk according to the present invention, thegroove is formed so that the optical depth becomes λ/4 to λ/8, and themagneto-optical recording film is formed so that the Kerr ellipticitythereof becomes approx. 0°. As a consequence, the cross talk can bereduced when the data, which were recorded on both the land and thegroove of the magneto-optical disk, are reproduced.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1(A) to 1(C) are schematic diagrams showing the structure of amagneto-optical disk according to an embodiment of the presentinvention. FIG. 1(A) is a diagram showing the structure of the land andthe groove of the magneto-optical disk according to the embodiment ofthe present invention. FIG. 1(B) is a sectional view showing the sectionof a part of the magneto-optical disk according to the presentinvention. And FIG. 1(C) is a diagram showing the structure of therecording film of the magneto-optical disk of the present invention;

FIG. 2 is a diagram showing the structure of a head unit according to anembodiment of the present invention.

FIG. 3 is a characteristic diagram showing cross talk characteristicswhich were measured with various Kerr ellipticities.

FIGS. 4(A) to 4(C) are characteristic diagrams showing the values of thecross talk which were obtained by calculation and the values of thecross talk which were measured by experiment.

FIGS. 5(A) and 5(B) are characteristic diagrams showing the values ofthe cross talk which were obtained by calculation and the values of thecross talk which were measured with a λ/12 board as a phase compensatingboard. FIG. 5(A) is a characteristic diagram showing the values of thecross talk which were obtained by calculation and the values of thecross talk which were measured without using the λ/12 board. And FIG.5(B) is a characteristic diagram showing the values of the cross talkwhich were obtained by calculation and the values of the cross talkwhich were measured using the λ/12 board.

FIG. 6 is a characteristic diagram showing the relationship between anormalized value and cross talk, the normalized value being obtained bynormalizing the intensity of a Gaussian beam incident on an object lens,obtained at the end of the object lens, with the intensity at the centerof the object lens.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, a description will be made in detail of a magneto-opticalrecording medium and a head unit for a magneto-optical recording mediumaccording to the present invention, accompanying with the drawings. Inan embodiment described hereinafter, a magneto-optical disk is taken anddescribed as an example of the magneto-optical recording medium.

FIGS. 1(A) to 1(C) show the structure of the magneto-optical diskaccording to the embodiment of the present invention. Themagneto-optical disk 1 has a disk substrate 4 having lighttransmittivity, a magneto-optical recording film 5, and a protectivefilm 6, as shown in FIG. 1(B). The disk substrate 4 is formed into adisk shape of thickness 1.2 nm with a synthetic resin material such aspolycarbonate or PMMA, and on one surface of the substrate 4, grooves 3are concentrically or spirally formed. As shown in FIG. 1(A), an areabetween the grooves 3 becomes a land 2. In the magneto-optical disk 1,the tracking control of the light beam, irradiated on themagneto-optical disk 1, is performed based on the light reflected fromthe groove, and data is recorded on both the land 2 and the groove 3.For the data that was recorded on the magneto-optical disk 1, as withthe aforementioned recording, the tracking control of the light beam,irradiated on the magneto-optical disk 1, is performed based on thelight reflected from the groove, and the data which was recorded on theland 2 or the groove 3 is read out. At this time, the qualities of thesignals read from the land 2 and the groove 3 can be made equal to eachother by forming the widths of the land 2 and the groove 3 so as tobecome 1:1. The optical depth d₀ of the groove 3 is given by thefollowing equation: ##EQU1## where λ is the wavelength of the light beamirradiated to the magneto-optical disk 1, "n" is the index of refractionof the disk substrate 4, "d" is the physical depth of the groove 3. Theequation giving the aforementioned d₀ is normalized with the wavelengthλ of the light beam irradiated to the magneto-optical disk 1.

The magneto-optical recording film 5 is formed on the surface where theland 2 and the groove 3 of the disk substrate 4 are formed by depositionand sputtering. The magneto-optical recording film 5, as shown in FIG.1(C), is composed of a plurality of films and formed on the disksubstrate 4 so that a first film, a second film, a third film, and afourth film are layered in this order from the side on which the lightbeam is incident. The first film 5a is composed of SiN and formed, onsaid one surface where the land 2 and the groove 3 of the disk substrate4 are formed, so that the film thickness becomes 800 Å. The second film5b is formed on the first film 5a with a vertical recording materialsuch as TbFeCo so that the film thickness becomes 200 Å. The third film5c is formed on the second film 5b with SiN so that the film thicknessbecomes 230 Å. The fourth film 5d is formed on the third film 5c withmaterial having a high reflectance such as Al so that the film thicknessbecomes 600 Å. If the magneto-optical disk 5 is thus formed, the Kerrellipticity ε_(k) can be made 0°, for example, when the wavelength λ ofthe light beam which is irradiated to the magneto-optical disk 1 is made780 nm.

The protective film 6 is formed on the fourth film 5d of themagneto-optical recording film 5 with synthetic resin material such asultraviolet hardening resin so that the protective film 6 covers themagneto-optical recording film 5 to protect the film 5.

The light beam in the state focused by a head unit such as describedlater is irradiated on the magneto-optical recording film 5 through thedisk substrate 4. Also, in recording data on the magneto-optical disk 1,a vertical magnetic field is applied from the protective film 6 side byan external magnetic-field generation unit (not shown), and at the sametime the light beam with a recording level is irradiated on themagneto-optical recording film 5 through the disk substrate 4, asdescribed above. With the light beam irradiated on the magneto-opticalrecording film 5 and the vertical magnetic field applied from theprotective film 6 side, data is recorded on the magneto-optical disk 1.The data which was recorded on the magneto-optical disk 1 is read outwith Kerr effect. Specifically, the light beam with a level lower thanthe recording level is irradiated via the disk substrate 4, and theirradiated light beam is reflected by the magneto-optical recording film4. As this occurs, the plane of linear polarization of the light beam isrotated depending upon the direction of magnetization of themagneto-optical recording film 4. By making use of the rotationalphenomenon of the linear polarization plane of the light beam irradiatedon the magneto-optical disk 1, the data which was recorded on themagneto-optical recording film 5 of the magneto-optical disk 1 is readout.

As described above, in the magneto-optical disk 1 according to thepresent invention, the Kerr ellipticity of the magneto-optical recordingfilm 5 is formed so as to become 0°, and as described later, the opticaldepth of the groove 3 is formed so as to become λ/4 to λ/8.Particularly, in this embodiment, the optical depth of the groove 3 isformed so as to become approx. λ/6. As a consequence, the cross talk canbe reduced when the data which were recorded on both the land 2 and thegroove 3 of the magneto-optical disk 1 are reproduced.

Now, with FIG. 2 a description will be made of the constitution of ahead unit 10 for the magneto-optical disk according to the embodiment.

In FIG. 2, the head unit 10 has a light source 11, a collimator lens 12,a beam splitter 13, an object lens 14, a λ/2 wavelength board 15, apolarization beam splitter 16, focus lenses 17 and 18, photodetectors 19and 20, and a phase compensating board 21. The light source 11 isconstituted by a semiconductor laser device, etc. The collimator lens 12converts a light beam emitted and emanated from the light source to acollimated light beam. The beam splitter 13 is formed into a cubic shapeby bonding a pair of trigonal optical prisms together and separates thelight beam emitted from the light source 11 from the light beamreflected by the magneto-optical recording film 5 of the magneto-opticaldisk 1. In this embodiment, the light beam reflected by themagneto-optical disk 1 is reflected 90° at the bonded surface of a pairof optical prisms of the beam splitter 13 and therefore is separatedfrom the light beam emitted from the light source 11. The object lens 14focuses the light beam, emitted from the light source 11 through thebeam splitter 13, on the magneto-optical recording film 5 of themagneto-optical disk 1 through the disk substrate 4. The object lens 14is driven by means of an actuator (not shown) in the optical axisdirection, i.e., focusing direction, and in the plane directionorthogonally crossing to the optical axis, i.e., tracking direction. Theobject lens 14 is composed, for example, of a single unspherical lens.The λ/2 wavelength board 15 rotates the plane of polarization of thereflection light from the magneto-optical disk 1, which was split by thebeam splitter 13, by 90°. The polarization beam splitter 16 splits thereflection light from the magneto-optical disk 1, which was split by thebeam splitter 13 and passed through the λ/2 wavelength board 15, intotwo light beams, a P-polarized light component and an S-polarized lightcomponent. The light beam of the P-polarized light component of the twolight beams, the P-polarized light component and the S-polarized lightcomponent split by the polarization beam splitter 16, is focused to thephotodetector 19 through the focus lens 17. Also, the light beam of theS-polarized light component is focused to the photodetector 20 throughthe focus lens 18. By taking a difference between the output signal fromthe photodetector 19 and the output signal from the photodetector 20,the reproduction signal of the data that was recorded on themagneto-optical disk 1 is generated. In addition, with the outputsignals from the photodetectors 19 and 20, there is generated an errorsignal, such as a focusing error signal and a tracking error signal fordriving the actuator in the focusing and tracking directions whichdrives the aforementioned object lens 14.

As the phase compensating board 21, a wavelength board is employed. Thisphase compensating board 21 has a retardation so that a phase (tan⁻¹(ε_(k) /θ_(k))) occurring due to the Kerr ellipticity ε_(k) is canceled.That is, if the phase compensation difference of the phase compensatingboard 21 is expressed by δ, the phase difference δ will be expressed bythe following equation: ##EQU2## where ε_(k) is the Kerr ellipticity andθ_(k) is the Kerr rotational angle.

In the phase compensation method using this wavelength board, phasecompensation is performed as follows. In a case where there is employeda wavelength board having the same retardation as the phase (tan⁻¹(ε_(k) /θ_(k))) occurring due to the Kerr ellipticity ε_(k) of themagneto-optical recording film 5 of the magneto-optical disk 1, thephase compensation of the reflection light from the magneto-optical disk1 is performed by arranging the wavelength board comprising the phasecompensating board 21 so that the crystal optical axis of the boardbecomes parallel to the plane of polarization of an incident light beam(in this embodiment, a light beam reflected from the magneto-opticaldisk 1, which was split by the beam splitter 13). Also, in a case wherethere is employed a wavelength board having a retardation greater thanthe phase (tan⁻¹ (ε_(k) /θ_(k))) caused by the Kerr ellipticity ε_(k) ofthe magneto-optical recording film 5 of the magneto-optical disk 1, thephase compensation of the reflection light from the magneto-optical disk1 is performed by rotating the optical axis of the wavelength board.With the phase compensating board 21 such as this, the Kerr ellipticityε_(k), caused by the Kerr effect of the light beam irradiated to themagneto-optical disk 1, can be compensated and at the same time thecross talk, which occurs in reading data that was recorded on themagneto-optical disk 1, can be reduced. As a consequence, there isobtainable a cross talk characteristic which is equivalent to thatobtained when the Kerr ellipticity ε_(k) is 0°.

Now, the case of the head unit 10 of the present invention using thephase compensating board 21 will be compared and described with the caseusing no phase compensating board 21.

Initially, in the case using no phase compensating board 21, the resultswhich were calculated by varying the Kerr ellipticity ε_(k) to 0°,-0.2°, and -0.5° are shown in FIG. 3. At this time, assume the Kerrrotational angle θ_(k) has been fixed to 0.9°. The leakage on the groove3 of a signal from the land 2 is referred to as "cross talk on groove"and is shown by "on groove" in FIG. 3. The leakage on the land 2 of asignal from the groove 3 is referred to as "cross talk on land" and isshown by "on land" in FIG. 3. In FIG. 3, there is shown the case of theKerr ellipticity ε_(k) having a minus sign. On the contrary, in the caseof the Kerr ellipticity ε_(k) having a plus sign, "the cross talk ongroove" in the case of the Kerr ellipticity ε_(k) having a minus signbecomes "the cross talk on land" in the case of the Kerr ellipticityε_(k) having a plus sign, and "the cross talk on land" in the case ofthe Kerr ellipticity ε_(k) having a minus sign becomes "the cross talkon groove" in the case of the Kerr ellipticity ε_(k) having a plus sign.For the other calculation conditions, the mark length of the signalwhich was recorded on the magneto-optical disk 1 is 1 μm, the widths ofthe land 2 and the groove 3 each are 0.8 μm, the wavelength λ of thelight beam, irradiated on the magneto-optical disk 1, is 0.78 μm, andthe numerical aperture (NA) of the object lens is 0.53.

If in FIG. 3 the Kerr ellipticity ε_(k) is 0°, then "the cross talk onland" and "the cross talk on groove" will become nearly equal to eachother and the cross talk will become minimum when the physical depth ofthe groove is 90 nm, i.e., the optical depth of the groove is about λ/6.If the Kerr ellipticity ε_(k) is not 0°, then the groove depths, where"the cross talk on land" and "the cross talk on groove" become minimum,will differ. If the absolute value of the Kerr ellipticity ε_(k) becomesgreater, then the depths where the cross talks become minimum will tendto greatly differ. As shown in FIG. 3, in order to make "the cross talkon land" and "the cross talk on groove" minimum at the same time, theKerr ellipticity ε_(k) of the magneto-optical recording film 5 of themagneto-optical disk 1 needs to be 0° and at the same time the opticaldepth of the groove 3 needs to be about λ/6. As a consequence, becausethe cross talk can be render minimum, it becomes possible to record dataon both the groove and the land and the recording density can be thusenhanced.

Subsequently, the case of the head unit 10 having the phase compensatingboard 21 will be described with an experimental example. In FIGS. 4(A)to 4(C), there are shown the results of measurement of the cross talk,which were obtained when a signal with a predetermined mark length wasrecorded at the Kerr rotational angle θ_(k) and Kerr ellipticity ε_(k)of 1.23° and 0.1°, 1.21° and -0.32°, and 1.33° and 0.49° with amagneto-optical disk having various kinds of groove depths. In FIGS.4(A) to 4(C), "" represents the measured value of "the cross talk onland" and "o" represents the measured value of "the cross talk ongroove." The solid line and broken line in FIGS. 4(A) to 4(C) show theresults which were obtained based on the conditions that were used inFIG. 3 by calculation. As a consequence, as evident in FIGS. 4(A) to4(C), the result of calculation and the measurement by experiment arenearly equal to each other. As shown in FIG. 4(A), when the Kerrellipticity ε_(k) is 0.1° and the physical depth of the groove is 90 nm,"the cross talk on land" and "the cross talk on groove" can be reducedto less than -35 dB at the same time. This cross talk value is a valueenough to reproduce the digital signal or digital data which wasrecorded on the magneto-optical disk serving as a recording medium. Aswith the result of calculation of FIG. 3, as shown in FIGS. 4(B) and4(C), even for the results obtained by experiment, as the Kerrellipticity ε_(k) becomes greater, the groove depths, where "the crosstalk on land" and "the cross talk on groove" become minimum, differ.

Next, the cross talk of the magneto-optical disk where the Kerrellipticity ε_(k) exceeds 0.1° was measured by experiment by using aλ/12 board as the phase compensating board 21. The results of theexperiment are shown in FIGS. 5(A) and 5(B). The experimental resultsshown in FIGS. 5(A) and 5(B) were obtained when the Kerr rotationalangle θ_(k) of the magneto-optical recording film of the magneto-opticaldisk is 1.26° and the Kerr ellipticity ε_(k) is -0.73°. FIG. 5(A) showsthe case where the λ/12 board as the phase compensating board 21 was notemployed. FIG. 5(B) shows the case where the λ/12 board was employed andwhere the λ/12 board was inserted and arranged in the optical system ofthe head unit so that the crystal optical axis of the λ/12 board becomesparallel to the linear polarization of the incident light beam. Thesolid line, the broken line, "," and "o" in FIGS. 5(A) and 5(B), aswith FIGS. 4(A) to 4(C), represent the results of the calculation andthe values of measurement of "the cross talk on land" and "the crosstalk on groove." The solid line in FIG. 5(B) represents values ofcalculation which were obtained when the Kerr ellipticity ε_(k) is 0°.

As shown in FIG. 5(A), in the case where the λ/12 board was notemployed, the physical depths of the groove, where "the cross talk onland" and "the cross talk on groove" become minimum, differ. On theother hand, in the case where the λ/12 board was employed, the value ofmeasurement nearly matches with the value of calculation when the Kerrellipticity ε_(k) is 0°, and the cross talk becomes minimum when thephysical depth of the groove is 90 nm. In a recording and/or reproducingdevice using an already standardized magneto-optical disk, the crosstalk value is required to be less than -26 dB. In this embodiment, therange of the physical depth of the groove, where the aforementionedcross talk value of less than -26 dB is obtained in recording and/orreproducing a digital signal or digital data on or from amagneto-optical disk, becomes a range from about 80 nm to 95 nm. This isbecause the λ/12 board has a retardation of 30° and shifts the Kerrellipticity ε_(k) of an incident light beam by a phase of 0.1°.

As described above, when the Kerr ellipticity ε_(k) shown in FIG. 5(B)is 0°, the calculated value and the measured value of the cross talknearly match with each other. In other words, it is shown that the depthdependence of the cross talk in the case of the Kerr ellipticity ε_(k)being 0° is obtained by employing a wavelength board having retardationso that a phase occurring due to the Kerr ellipticity ε_(k) is canceled.

As described above, the optical depth of the groove 3 of themagneto-optical disk 1 is formed so as to become approx. λ/6, and themagneto-optical recording film 5 is formed with material or a multilayerso that the Kerr ellipticity ε_(k) becomes 0°. With this, even in thecase data was recorded on both the land 2 and the groove 3, the crosstalk leaking from the land or the groove can be reduced and therecording density of the magneto-optical disk 1 can be thus enhanced, inreading data from the land 2 and the groove 3.

However, the optical depth of the groove 3 does not always need to beabout λ/6. As described below, similar effects can be obtained even whenthe lower limit of the optical depth of the groove 3 is about λ/8 andalso the upper limit is less than λ/4. The result of the experiment isshown in FIG. 6. In FIG. 6, RI represents values which were obtained bynormalizing the intensity at the object lens end of the disk in radialdirection of a Gaussian beam incident on the object lens by means of theintensity of the Gaussian beam at the center of the object lens, and thecases of RI being 0.05, 0.18, 0.33, and 0.55 are shown. For theconditions of this experiment, the wavelength λ of a beam light,irradiated to the magneto-optical disk, is 780 nm, the numericalaperture NA of the object lens is 0.53, the mark length of a signal,recorded on the magneto-optical disk, is 1 μm, and the widths of theland and the groove each are 0.8 μm. As evident in FIG. 6, the physicaldepth of the groove where the width of the cross talk becomes minimumdepends upon the value of RI. As described above, in recording a digitalsignal or digital data on the magneto-optical disk, the range of thephysical depth of the groove where the cross talk of less than -26 dB isobtained becomes a range where the lower limit is about 65 nm and theupper limit is less than 130 nm, as shown in FIG. 6. This is equivalentto the case where the lower limit of the optical depth of the groove isabout λ/8 and the upper limit is less than λ/4.

Also, in the head unit 10 according to the aforementioned embodiment,the phase compensating board 21 comprising a wavelength board isinserted and arranged between the beam splitter 13 and the λ/2 board 15,thereby compensating the Kerr ellipticity ε_(k) of the magneto-opticalrecording film 5 of the magneto-optical disk 1 where the optical depthof the groove 3 is formed to about λ/6. As a consequence, as with thecase where the Kerr ellipticity ε_(k) of the magneto-optical recordingfilm 5 of the magneto-optical disk 1 is not 0°, even when data has beenrecorded on both the land 2 and the groove 3, the cross talk can bereduced in reading data from the magneto-optical disk 1. Consequently,because data can be recorded on both the groove and the land of themagneto-optical disk 1, the recording density of the magneto-opticaldisk 1 can be enhanced.

In the present invention described above, an example of themagneto-optical disk, where grooves are concentrically or spirallyformed, has been described as a magneto-optical recording medium. Thepresent invention is not limited to a magneto-optical disk such as this,but it is also applicable to a magneto-optical disk where a plurality ofpre-pits for detecting a tracking error are formed instead of grooves.In this case, in a digital signal or digital data recording areafollowing the pre-pit, grooves whose optical depth is about λ/6 havebeen previously formed across the center of a virtual track prescribedwith the pre-pit. With this, as with the magneto-optical disk of theaforementioned embodiment, an effect of reducing cross talk isobtainable.

Also, in the aforementioned head unit according to the presentinvention, the phase compensating board 21 has been inserted andarranged between the beam splitter 13 and the λ/2 board 15. The presentinvention is not limited to this, but the phase compensating board 21may be arranged and inserted in any position in a return optical pathbetween the object lens 14 and the polarization beam splitter 16. Inthis case, the optical axis of the phase compensating board 21 withrespect to an incident light beam is regulated according to the positionwhere the phase compensating board 21 is arranged and inserted. Withthis, the same effect as that of the head unit of the aforementionedembodiment is obtainable. In addition, the phase compensating board 21is not limited to the wavelength board. Even when there is employed anoptical phase delay device having retardation so that a phase (tan⁻¹(ε_(k) /θ_(k))) caused by the Kerr ellipticity ε_(k) is canceled, asimilar effect is obtainable.

I claim:
 1. A magneto-optical recording medium comprising:a substrateformed from material having light transmittivity; and a magneto-opticalrecording film provided on one surface of said substrate; wherein insaid one surface of said substrate where said magneto-optical recordingfilm is provided, when a wavelength of a light beam, irradiated on saidmagneto-optical recording medium, is λ, there are formed grooves havingan optical depth where its lower limit is λ/8 and its upper limit isλ/4, and said magneto-optical recording film is formed so that the Kerrellipticity thereof becomes 0°.
 2. The magneto-optical recording mediumaccording to claim 1, whereinsaid grooves are formed so as to have anoptical depth of about 1/6 of the wavelength λ of the light beam,irradiated on said magneto-optical recording medium.
 3. Themagneto-optical recording medium according to claim 1, whereinsaidgrooves are formed so that a width of said groove and a width of a landbetween said grooves become nearly equal to each other.